Wasted time, effort, and material occurs in automated milling and cutting factories due to the accidental use of cutting tools and cutting bits of incorrect sizes. Typically, such tools and bits are loaded into compartments in a storage magazine by an operator, each compartment being intended to contain a tool of designated size. A mistake by an operator can cause a tool of the wrong size to be inserted into one of the compartments. When the tools are withdrawn from the magazine by robotic mechanisms for loading into the chuck of a milling machine, the error in tool size may not be detected. Further, even if no error exists in the sizes of the tools, events following their removal can cause tool size variations to occur. Such events include natural wear and tear as well as accidental chipping and breaking. If these variations are not detected, the automated milling machinery will cut the pieces upon which it works to the wrong specifications, thus rendering them unusable.
Numerous approaches exist which attempt to detect these variations in tool size. One approach utilizes mechanical fingers which bracket a tool to be measured and thereby perform a caliper-like mensuration operation. Mechanical fingers suffer the disadvantage that they themselves are subject to wear and tear, and thus are subject to the problem of their own dimensional changes. Further, mechanical fingers, in order to work properly, must contact the tool to be measured, and consequently, they will apply forces to the tool which may move the tool to an undesired position. Further still, mechanical feeler gauges can include delicate precision mechanisms which are subject to the malfunctions common to all mechanical devices.
Another approach is to use a light source to illuminate the tool in question to thereby cast a shadow upon photosensitive elements which measure the size of the shadow and thus obtain an indication of the size of the tool. One problem which can occur in this approach is that refraction occurs at the edges of the tool and thus light rays traveling past the tool from these regions tend to diverge and thus cast a blurred image at the shadow's edge. The greater the distance of the shadow from the tool, the greater is this effect. To minimize this effect by reducing this distance poses the problem of positioning the photosensitive elements near the cutting region of the tool, which is a region subject to vibration, noise, and flying debris.